Some methods of measuring the concentration of a specific component in a sample liquid employ electrochemical techniques for example. In these methods, a reaction system is formed from a sample liquid, an oxidoreductase and an electron transporter for example, voltage is applied to this reaction system using electrodes, and the concentration of the specific component is computed based on the resulting response current. Such a reaction system is formed for example on a biosensor provided with a reagent part comprising an oxidoreductase and an electron transporter. Because an oxidation-reduction reaction occurs between the specific component and the electron transporter in the reaction system due to the catalytic effect of the oxidoreductase, the amount of the electron transporter which becomes reduced (or oxidized) reflects the concentration of the specific component. Response current is obtained in correlation with the amount of electron movement occurring in the reaction system. Consequently, the accuracy of concentration measurement is greatly affected by the accuracy of measurement of response current.
In such methods, when blood (blood containing blood cells) is used as a sample liquid, electron movement between the electrode and the electron transporter is impeded by blood cells present on the surface of the electrode. As a result, the measured response current is reduced as the number of blood cells increases, producing measurement errors. Moreover, if the proportion of blood cells in the blood (hematocrit value) is different the measured response current will be different even if the glucose concentration is the same.
To solve these problems, methods have been proposed of separating blood cells from blood in the measuring instrument. Methods of separating blood cells include for example a method of providing a separation membrane in that part of the measuring instrument where blood or other sample liquid is introduced (see for example JP-A 8-114539 and JP-A 2002-508698), and a method of covering the surface of the electrode with a polymer membrane (see for example JP-A 6-130023, JP-A 9-243591 and JP-A 2000-338076).
However, because in methods which filter blood cells in the measuring instrument the plasma component must pass through the separation membrane it takes a long time for the plasma to reach the surface of the electrode, prolonging the measurement time. This problem can be solved by securing a large quantity of whole blood to be used, but this places a heavy burden of blood collection on the user.